A. Polishing of metal layers in IC processing
Chemical Mechanical Polishing (CMP) includes both chemical reaction and mechanical abrasion. It involves the use of a polishing slurry which contains chemically reactive components which form soft compounds with the surface region of the material being polished. The slurry also contains abrasive particles which mechanically preferentially remove the softer reacted surface region when the wafer is moved across the polishing pad having slurry thereon.
As integrated circuits become smaller, higher density, and faster, the process technology for manufacture of these circuits has undergone major transitions. With device critical dimensions decreasing below 0.25 microns, a major factor in limiting circuit response times is the metal interconnections connecting devices and circuit elements. Hence, decreasing the resistance of the metal lines, and reducing the capacitance between metal lines, are of great importance in IC process development. Two areas of development have been: 1) Cu metallization used in place of Al or Al alloy metallization due to the lower resistivity and higher electromigration resistance of Cu, and 2) use of low dielectric constant (low K) dielectrics as insulation between metal layers to reduce the capacitive coupling.
Use of Cu in IC metallization presents challenges, including the difficulties in reactive ion etching (RIE) of Cu. An alternative strategy to eliminate the requirement of RIE for Cu metallization is to use Damascene structures. This technique involves depositing an insulating layer, patterning and etching trenches in the insulation where the metal lines are to be placed, depositing Cu over the whole surface and into the trenches, and finally polishing off the surface Cu using Chemical Mechanical Polishing (CMP), thereby leaving only the embedded Cu lines in the trench regions. Related methods such as dual Damascene can integrate via and interconnect formation.
Current CMP technology requires refinement of polishing methods in order to avoid problems such as recession and erosion, as illustrated in FIGS. 1a and 1b. Current CMP technology also exhibits other types of Within-Wafer Non-Uniformity (WIWNU) of polish rate which result in uneven material removal across the wafer.
B. Field responsive materials
Field responsive materials exhibit a rapid, reversible, and tunable transition from a liquid-like, free-flowing state to a solid-like state, upon the application of an external field. These materials demonstrate dramatic changes in their rheological behavior (viscosity) in response to an externally applied electric or magnetic field, and are known as electrorheological (ER) fluids or magnetorheological (MR) fluids respectively. ER fluids may include linear dielectric particles (such as: silica, alumina, titania, barium titanate, semiconductors, weakly conducting polymers, zeolites, polyhydric alcohol, sodium carboxymethyl Sephadex, alginic acid, carboxymethyl Sephadex, lithium hydrazinium sulfate, or combinations thereof) colloidally dispersed in nonconducting continuous phase liquids (such as: silicone oils, mineral oils, paraffin oils, hydraulic oils, transformer oils, perfluorinated polyethers, or combinations thereof). Alternatively, homogeneous liquid-crystalline polymer-based materials are known, as reported by Inoue et al in the MRS Bulletin, August 1998. MR fluids comprise ferromagnetic or ferrimagnetic, magnetically nonlinear particles (such as: iron, iron alloys, iron oxide, iron nitride, iron carbide, carbonyl iron, low carbon steel, silicon steel, chromium dioxide, fumed or pyrogenic silica, silica gel, titanium dioxide, magnetite, nickel, cobalt, manganese, zinc, ceramic ferrites, or combinations thereof) dispersed in an organic or aqueous continuous phase liquid (such as: H2O, silicone oils, kerosene, mineral oils, paraffin oils, hydraulic oils, transformer oils, halogenated aromatic liquids, halogenated paraffins, diesters, polyoxyalkylenes, fluorinated silicones, cornstarch, olefin oil, glycol, or combinations thereof). Both ER and MR fluids may additionally comprise surfactants (which act as wetting agents) and thixotropic additives (which make the particles hydrophilic). Details of continuous phase liquids, particulates, surfactants, and thixotropic additives used for MR and ER fluids are found in US Patents having the following serial numbers: U.S. Pat. No. 5645752, U.S. Pat. No. 5167850, U.S. Pat. No. 4992190, U.S. Pat. No. 4033892, USRE032573, U.S. Pat. No. 3917538, and U.S. Pat. No. 4772407. All of these aforementioned patents are hereby incorporated by reference.
A feature shared by the ER and MR fluids is that after an external field is applied, the material rapidly transforms from a fluid into a weak viscoelastic solid, generally through the formation of chains and columns of the field responsive particles. These field-induced chain-like structures possess a non-zero shear modulus and a shear stress.
ER and MR materials are of interest in, and have been investigated for, such applications as engine mounts, shock absorbers, clutches, seat dampers, variable-resistance exercise equipment, earthquake-resistant high-rise structures, positioning devices, and optical polishing of aspherical surfaces. The materials science of field-responsive fluids is described in the MRS Bulletin, August 1998.
Optical glass polishing using MR fluids has been described by Jacobs et al in U.S. Pat. No. 5,616,066, issued Apr. 1, 1997, and in U.S. Pat. No. 5,795,212, issued Aug. 18, 1998. According to these patents, portions of the glass workpiece are selectively polished by moving the workpiece through a work zone having a stiffened MR fluid therein and therefore a high pressure. The MR finishing machine comprises an electromagnet, a trough for MR fluid containment, and a work spindle to which the curved glass workpiece is mounted.
ER and MR materials have not, to the best knowledge of the inventor, been utilized or investigated in the prior art for the polishing of metal layers in semiconductor processing. It is believed that their utilization for this application will enable solutions to some of the aforementioned problems which exist in current Al and Cu CMP technology, including recession and erosion, and other types of WIWNU.